Examples of common rail fuel pumps of radial pump design are known from, for example, EP-B-1705368 and EP-A-2050952. FIG. 1 of the accompanying drawings is a sectional view of one known radial fuel pump, which will now be described to illustrate the prior art.
The pump 100 of FIG. 1 comprises three pumping plungers 102 that are arranged at equi-angularly spaced locations around an engine-driven cam 104. Each plunger 102 is mounted within a plunger bore 106 provided in the housings 107a of respective pump heads 107. The pump heads 107 are mounted to a main pump housing 108 of the pump 100.
As the cam 104 is driven in use, the plungers 102 are caused to reciprocate within their bores 106 in a phased, cyclical manner. As the plungers 102 reciprocate, each causes pressurisation of fuel within a pump chamber 109 defined at one end of the associated plunger bore 106. The delivery of fuel from the pump chambers to a common high pressure supply line (not shown) is controlled by means of delivery valves (not shown). The high pressure line supplies fuel to a common rail, or other accumulator volume, for delivery to downstream injectors of a common rail fuel system.
The cam 104 carries a cam ring, or cam rider 110, which is provided with a plurality of flats 112, one for each plunger 102. An intermediate member in the form of a tappet 114 co-operates with each of the flats 112 on the cam rider 110 and couples to an associated plunger 102 so that, as the tappet 114 is driven upon rotation of the cam 104, drive is imparted to the plunger 102. As each tappet 114 is driven radially outward, its respective plunger 102 is driven to reduce the volume of the pump chamber. This part of the pumping cycle is referred to as the pumping stroke of the plunger 102, during which fuel within the associated pump chamber is pressurised to a relatively high level.
As the rider 110 rides over the cam 104 to impart drive to the tappets 114 in an axial direction, a base surface of each tappet 114 is caused to translate laterally over a co-operating region of an associated flat 112 of the rider 110. This translation of the tappets 114 with respect to the rider 110 causes frictional wear of the tappets 114 and the rider 110. Frictional wear particularly occurs at lateral edges of the tappets 114.
The rider 110 tends to turn on its axis during operation, so that the flats 112 tend to move away from perpendicular with respect to the axes of the respective pumping plungers 102. This means that the base surfaces of the tappets 114 tend to meet the flats at an inclined angle. This gives rise to an edge contact between the tappets 114 and the rider 110, which can exacerbate the problem of frictional wear. In particular, the edge contact results in a local temperature increase, which undesirably heats other components within the fuel pump assembly.
Due to the turning movement of the rider 110, the tappets 114 experience a torque which in turn gives rise to side loads that act on the plungers 102. As a result, frictional wear also occurs where each plunger 102 engages its respective tappet 114. The plungers 102 are guided in the bores 106, so the torque acting on the tappets 114 causes the tappets 114 to become inclined with respect to the plungers 102. The contact between the end of each plunger 102 and the corresponding tappet 114 is therefore also an edge contact, which can again lead to a high wear rate and localised heat generation.
The side loads acting on the plungers 102 also give rise to wear at the interfaces between the plungers 102 and the bores 106 in the head housings 107a. Wear at the plunger-bore interface can result in loss of volumetric efficiency of the pump, and in severe cases in plunger seizure and loss of pumping function.
An additional problem that arises when wear occurs between the rider 110 and the tappets 114, between the tappets 114 and the plungers 102, and between the plungers 102 and the head bores 106 is that wear debris can be produced. If such debris becomes entrained at an interface, for example between the tappet 114 and the rider 110, a dramatic increase in the wear rate can occur, which can lead to catastrophic failure of the pump.
It is known in some fuel pumps to omit the tappets, and instead to provide pumping plungers with integral interface members in the form of feet as described in, for example, EP-A-2048359. In these cases, similar wear problems to those described above arise at the interfaces between the plungers and the bores, and between the plunger feet and the rider flats.
It is known in the prior art to use fuel to lubricate the side contact surfaces of fuel pump plungers. For example, JP 2002 276508 describes a fuel pump in which a pumping plunger is provided with grooves to direct fuel from a fuel inlet passage to lubricate side contact surfaces of the plunger. EP-A-2088309 describes a fuel pump in which fuel can leak from the pump chamber between a plunger and its corresponding bore, providing a degree of lubrication to the side contact surfaces, and an arrangement of passages is provided in the pump housing to allow the leakage fuel to return to drain.
Against this background, it would be desirable to provide a fuel pump assembly in which the above-mentioned problems are reduced or mitigated.